1. Field of the Invention
The present invention relates to a workpiece cleaning apparatus and, more particularly, to a workpiece cleaning apparatus using a dense phase fluid.
2. Prior Art
Conventional solvent based cleaning processes are continuously being looked at for alternative techniques to reduce the ill effects placed on the environment in the form of water and air pollution as well as ozone depletion. Government legislation continues to be introduced to protect the environment. Accordingly, there is an ever increasing desire to find alternative non-polluting methods of solvent based processing. The use of dense phase gases and mixtures of gases and cosolvents or surfactants is being explored. The solvent properties of compressed gases is well known. In the late 1800's, Hannay and Hogarth found that inorganic salts could be dissolved in supercritical ethanol and ether. Buchner discovered by the early 1900's, that the solubility of organics such as naphthalene and phenols in supercritical carbon dioxide increased with pressure. Within forty years Francis had established a large solubility database for liquefied carbon dioxide which showed that many organic compounds were completely miscible.
One industry that employs extensive conventional solvent based cleaning processes is semiconductor manufacturing. Future restrictive legislation on conventional solvent based processes would thus have a significant adverse impact on the industry, and ultimately on the ability of consumers to have affordable electronic devices which are desired in ever increasing numbers. By way of example, the conventional process to manufacture a semiconductor device or workpiece generally involves a silicon wafer that undergoes numerous processing steps where materials are deposited in and on the wafer. Through this repetitive processing, electrical circuits are created within the wafer. As part of the manufacturing process there are a number of cleaning steps where conventional predominately aqueous based chemistry is used to perform the “cleaning” of the surface and film at each particular step. The chemistry is selected based on its performance on the exposed structures. Conventional wafer cleaning generally employs a batch processing method due in part to the time involved to ensure that the wafer surface is sufficiently cleaned, and the possibility of minimizing the amount of conventional solvent that may be used during the cleaning process. In view of the extensive use of conventional solvents in semiconductor manufacturing, the future restrictions on use of conventional solvents, as well as the uncertainty as to the time frame and extent of those restrictions, has a major adverse impact on the industry. Accordingly, the industry desires alternatives to conventional aqueous based chemistries such as using dense phase fluids or a mixture of dense phase fluids and cosolvents for semiconductor wafer cleaning.
There are a number of consideration when seeking alternatives to conventional cleaning methods. A major consideration is the desire for the throughput rate to be as high as possible and at least equal to throughput rates of conventional processes. Generally, this can be accomplished in one of two modes. The first is a batch processing method that would be similar to the existing aqueous based equipment solutions. An alternative would be to insert the processing solution just after the existing film processor. This insertion point becomes advantageous when the typical film processors in the industry operate on a single workpiece throughput integrated. Currently, there is a desire to operate on a single workpiece process and move away from batch processing in the industry. The introduction of dense phase gases processors has the potential to be easily integrated into the desired semiconductor tooling methodology as a single workpiece processor. The challenge in this case is that it is desired that processing rates for pre film deposition cleaning and post film deposition cleaning be on the same order as the film deposition. Conventional cleaning methods using alternative solvents have however fallen well short of this goal.
U.S. Pat. No. 5,013,366 describes a cleaning process for removing contamination from a substrate wherein the substrate to be cleaned is contacted with a dense phase gas at a pressure equal to or above the critical pressure of the dense phase gas. The phase of the dense phase gases is then shifted between the liquid state and the supercritical state by varying the temperature of the dense phase fluid in a series of steps between temperatures above and below the critical temperature of the dense fluid. At each step in the temperature change, the dense phase gas possesses different cohesive energy density or solubility properties. Temperature control of the supercritical process is performed through applying the thermal changes to the processing apparatus. In this case it is applied to a large thermal mass (high pressure vessel) and as a result a considerable amount of time is used for the apparatus to achieve the target process parameters.
U.S. Pat. No. 5,261,965 describes a method and system which is based on first cooling the semiconductor wafer to a predetermined temperature in order to condense a liquid film on the semiconductor wafer surface from a condensable process gas or gas mixture. Then, the method and system promote thermally activated surface reactions and rapidly evaporate liquid film from the semiconductor wafer surface using a high peak power, short pulse duration energy source such as a pulsed microwave source to dissolve surface contaminates and produce drag forces sufficiently large to remove particulates and other surface contaminates from the surface of the semiconductor wafer. Although, the system in this case attempts to improve process times, and hence increase system throughput, the resulting system is highly complex and costly. The present invention overcomes the problems of the prior art as will be described in further detail below.